I am using Ubuntu 12.04 in my HP laptop (A8 processor), It is getting overheated when I use Ubuntu, but when I use Windows it stays cool. I want to switch off my graphics card in Ubuntu. This has to be applied when I shutdown or restart. Currently I'm getting an error saying that GNOME settings not responding and then it is taking long time to shutdown the system.

Check BIOS settings. There could be option to use integrated graphics, instead of graphic card, of-course only if you have on board graphics on motherboard. There could be graphical way or from within Ubuntu, but best thing would be to use BIOS settings.